So what exactly is NATO, and why is the organization so significant? ABC News breaks down the alliance.

What is NATO?

NATO stands for the North Atlantic Treaty Organization, a security alliance established in 1949 during the early days of the Cold War to counter Soviet aggression in Europe.Now numbering 29 countries in Europe and North America, the alliance’s goal is to “safeguard the freedom and security of its members through political and military means,” according to its website.

North Macedonia is on track to become the organization’s 30th member.

The organization promotes “democratic values” and encourages member nations to work together on issues of defense and security to prevent long-term conflict.

NATO adheres to a policy of collective defense, meaning an attack on one member is considered \"an attack against all.\" The policy is outlined in Article 5 and has only been invoked once, after the Twin Towers in New York City were attacked Sept. 11, 2001, and NATO members sent troops to Afghanistan where they still train and advise Afghan security forces today. There have been 1,044 non-U.S. NATO service members killed fighting in Afghanistan.

How does NATO work?

Headquartered in Brussels, Belgium, each member nation is represented by an ambassador that sits on the North Atlantic Council (NAC), the alliance’s political decision-making body. The NAC meets at least once a week and is chaired by Stoltenberg, a former prime minister of Norway.

When political decisions require military involvement, NATO’s Military Committee helps plan the military elements needed for an operation. While NATO has few permanent military forces, member nations can voluntarily contribute forces when the need arises.

Where is NATO operating right now?

Currently, NATO’s website lists six active operations and missions: Afghanistan, Kosovo, securing the Mediterranean Sea, training mission in Iraq, supporting the African Union, and policing airspace.

ABC's Elizabeth McLaughlin contributed to this report from the Pentagon.

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